Process of recovering sodium nitrate from liquors containing it and sodium chlorid in solution.



At'tys.

I. B. HOBSBAWM & J. L. GRIGIONI. PROCESS OF RECOVERING somum NITRATE FROM uouons CONTAINING IT AND SODIUM CHLORID IN SOLUTION.

APPLICATION FILED MAY 20, 1915.

Patented June 19, 1917.

l \i 7 g i m smw w wmm o r. 3 W 88 8 m M BMW ISIDORE BERKWOOD HOIBSIBAWM, OF LONDON, AND JAMES L. GRIG-IONI,

ENGLAND.

ATES PATENT FFTCE;

or RICHMOND,

PROCESS OF RECOVERING SODIUM NITRATE FROM LIQUORS CONTAINING IT AND SODIUM CHLORID IN SOLUTION.

Specification of Letters Patent. Patented June 19, 1191]. 7.

Application filed May 20, 1915. Serial No. 29,444.

evaporation process for recovering sodium nitrate from liquors containing it and sodium clllOrid in solution, and an apparatus for carrying out such process.

The liquors or solutions which we have more particularly in mind are those obtained by the action of water on caliches, costras or any other form of naturally occurring sodium nitrate and sodium chlorid containing substances, as well as the ripios- 0r refuse dumpings from old or present nitrate workings.v

In the present state of development of the extractionof these naturally occurring nitrate containing substances it is found more economical not to work to the fully concentrated liquor in the lixiviation plant, but to work to a weaker liquor than formerly and subsequently concentrate by evaporation.

These liquors have hitherto been brought up to full crystallizing strength on a commercial scale by concentrating to the convenient gravity at which "point the salt is rendered'partially insoluble. This gravity can be considered purely empirical and depends entirely on local conditions and not at all on the ultimate chemical composition of the liquor.

For this reason the sodium chlorid remaining in solution becomes a variable quantity. The liquors with the salt in suspension are subsequently run off into settling tanks wherein the bulk of the sodium chlorid is allowed to settle to the bottom by the dual agency vof gravity and the. cooling of the liquor The supernatant liquor, is then decanted off into crystallization tanks in which by further cooling of the liquor the sodium nitrate is crystallized.

Such a method of dealing with the liquor is altogether unsatisfactory for the following among other reasons:

1) The supernatant liquor which is run off into the crystallization tanks usually contains a fairly considerable and always indefinite percentage of the sodium chlorid in solution which may be deleterious or otherwise that'is to say no two batches of liquor are alike;

(2) Owing to the slowness of the deposition of the sodium chlorid in the settling tanks and further also owing to the fact that the temperature of the liquor while in said tanks is substantially reduced, an unduly large percentage of the sodium nitrate crystallizes and settles with the sodium chlorid;

(3) There is no. continuity in the evaporation processes heretofore practised. I

(4) The crystallized sodium nitrate can not be obtained in a less period than 1i to 16 days.

It has also been known for many years that by merely boiling a liquor containing sodium nitrate and sodium chlorid some of the latter will be precipitated. The difficulty however which has existed was how to effect. the precipitation and separation of all the sodium chlorid by boiling the liquor without at the same time causing the deposition of sodium nitrate.

The main objects of our invention are to enable by a single operation (1) substan= tially the whole of the sodium nitrate to be recovered; (2) the sodium nitrate to be recovered either practically free from or together with any desired predetermined percentage of sodium chlorid; the process to be carried on continuously; and (4) the crystallized sodium nitrate to be obtained in a few hours.

If the sodium nitrate is to be recovered practically free from sodium chlorid, our process, described general terms, consists in raising the temperature of and concentrating liquor containing both sodium nitrate and sodium chlorid in solution, until the concentration has been carried to such a degree that the boiling point of the liquor at normal or atmospheric pressure '21. e. at a pressure of 14.7 lbs. absolute per square inch (when the liquor contains only pure sodium chlorid and pure sodium nitrate) is about tion, and the liquor must be maintained at this temperature while the sodium chlorid is settling.

The supernatant liqu0r-Which has thus been practically completely freed from sodium chloridis then removed and further evaporated whereby the sodium nitrate is crystallized.

If it is desired that the sodium nitrate recovered shall contain a predetermined percentage of sodium chlorid, it will not be necessary to heat the liquor to the actual critical temperature. The temperature to which the liquor should be raised will depend upon the particulartype of liquor to be treated, and also upon the percentage of sodium chlorid to be ultimately deposited with the sodium nitrate. Merely by way of example We may state that in our experiments and. trials we found that with a liquor having a critical temperature of 123 C. l per cent. of sodium chlorid remained in solution when the temperature of the liquor was raised to only 122 o.

The critical temperature referred to varies with the type or composition of liquor, and, of course, it will vary with the pressure at which the process is carried out. In our ex periments and trials we found that working with the liquor at atmospheric pressure 71. 6. 14.7 lbs. absolute per square inch, with one liquor the critical temperatupe was 122 C. and with another 120 C. this variation being caused by the presence in solution of a salt or salts other. than sodium chlorid and sodium nitrate. Either the critical temperature (at which liquor about to be treated on a commercial scale will be completely freed from sodium chlorid as aforesaid) or that other temperature (at which a desired predetermined percentage of the sodium chlorid will be allowed to remain in solution) is in practice to be determined by experiments on a sample of such liquor.

\ Such experiments or tests may be readily and quickly performed in the laboratory.

The maximum boilingpoint is determined by the chemist on a sample of the liquor in the method usually adopted for such determination that is to say :the liquor is boiled at a pressure of ll? lbs? absolute per square inch until the maximum temperature is registered and after freeing the liquor of the solids which have been deposited dur-.

ing the concentration, the maximum boiling point of the clear liquor is registered afresh, and this is the critical temperature.

Our process may be carried out with the .aid of two evaporators in the first of which the separationto the desired extentof the sodium chlorid is effected, the liquor (now either practically free from sodium chlorid or containing a desired or predetermined percentage thereof) being then transferred to the second evaporator in which it is further evaporated to bring about the crystallization of the sodium nitrateand the deposition therewith of the aforesaid predetermined percentage chlorid.

It will be obvious that the said evaporators may be of single or of multiple efi'ect this depending upon the amount of water to be removed from the liquors to be treated.

In view of the fact that the cost of fuel whereby ultimately the evaporation of the liquors is effected, forms a very important item in the total cost of recovering the sodium nitrate from such liquors it will be readily appreciated that the avoidance of waste of heat is of almost primary impor tance.

out on most economic lines we may so connect the two evaporators referred to that the hot vapors given off by the liquor when being concentrated in the first evaporator, to bring about the separation of the sodium chlorid as described, are utilized to produce the evaporation of the already desalted liquor in the second evaporator to thereby bring about the crystallization of the sodium nitrate, the boiling point of the liquor in said second evaporator being regulated by a difference in pressure in the two evaporators.

The accompanying drawing shows diagrammatically and merely by way of example one form of apparatus whereby our process may be carried out.

In said drawing the numeral '1 indicates an evaporator which comprises a calandria 2 and a separator 8, and 4 indicates another evaporator comprising a calandria 5 and separator 6.

The liquor to be treated is contained in one or more tanks 7 from which it is pumped by a pump 8 through a preliminary heater 9 in which the temperature of the liquor is raised to a convenient point. From this heater the liquor is led by pipes 10, 11, to the first evaporator in which it is heated and concentrated (live steam being admitted to the calandria 2 by a pipe12) the vapor given off during concentration escaping through a pipe 13. After a short time sodium chlorid will be precipitated and in order to trap this in the separator we provide the latter with an apron or funnel 14 which will compel the by the arrows 15 and insure the precipitated (if any) of sodium In order that our process may be carried liquor to circulate in the direction indicated sodium chlorid settling or depositing on the bottom of the separator.

When the concentration has been carried to such a degree that the boiling point of the liquor when working at a pressure of 14.7 lbs. absolute per square inch, is about 123 C. (more or less according to circumstances al ready referred to) practically all the sodium chlorid will have been precipitated and by maintaining the liquor at that temperature the sodium chlorid will settle as described.

The supernatant liquor having thus been desalted (i. e. freed from sodium chlorid) is conducted by a pipe '16, which ,connects to the separator 3 at the point 34, directly into the separator 6 of the second evaporator 4 in which it is further evaporated to cause the sodium nitrate to crystallize and be deposited on the bottom of the separator 6.

The evaporation in the evaporator 4 is carried on at a less pressure than is the concentration in the evaporator 1, so that the hot vapors from the evaporator 1 may be utilized-to bring about such evaporation and for this purpose the pipe 13 referred to is connected to the calandria 5.

As it will be generally convenient to work the evaporator 1 at pressure 71. e. a pressure of 14.7 lbs. absolute per square inch we make provision to work the evaporator 4 under a partial vacuum (to reduce the boiling point of the liquor) this being attained by an exhauster 17 and condenser 18.

The exhauster 17 will of course not be required iffor example at high altitudes the evaporation in the evaporator 4 is carried on at the there atmospheric pressure viz. at less pressure than 14.7 lbs absolute per square inch and the concentration in the evaporator 1 at a pressure over and above the there atmospheric viz. at 14.7 lbs. absolute per square inch.

The sodium chlorid deposited in the separator 3 may be removed therefrom in any suitable manner. For example we may provide a filter box 19 having a filter diaphragm 20 of wire gauze or other suitable material, the upper part of the box being connected to the bottom of the separator 3 by a pipe having a valve 21 and the lower part of the filter being connected by a pipe 22having a valve 23to the pipe 16.

By periodically opening the valve 21 the sodium chlorid at the bottom of the separator 3 together with some of the liquor will drop into the filter box. The sodium chlorid will be held up by the filtering medium 20 whereas the liquor Will pass therethrough and through the pipe 22 into the pipe 16 to be transferred to theevaporator 4, this being insured (the valve 21 having of course been closed) by allowing a little air or steam into the filter box. The nitrate liquor which,

adheres to the sodium chlorid in the filter box may be washed off by steam or hot water and the resultant liquor returned,'say, to the tank 7, or the sodium chlorid may first be removed and then ,Washed.

The filter box is provided with a door 25 through which the sodium chlorid may be removed and the filter cleaned.

The sodium nitrate which has deposited to the bottom of the separator 6 may also be removed in any suitable manner. In the construction of apparatus illustrated, we provide a box 26 which is connected to the bottom of the separator 6 by a pipe having a valve 27 said box communicating at its lower end by a pipe 28 having a valve 29 with a hydro-extractor, for example a centrifugal drier 30.

Periodically the attend-ant will open the valve 27 and thereby allow the crystallized nitrate together with a small volume of liquor in the separator 6 to drop into the box 26. The valve 27 is next closed and the valve 29 opened to allow the nitrate and liquor to drop or flow to the hydro-extractor where practically all the liquor will be extracted and the sodium nitrate now substantially dry will be removed in any suitable manner.

The liquor extracted is allowed to flow into a tank 31 from which it is pumped back into the first evaporator through a pipe 32 or back into the storage or liquor tank 7.

The flow of liquor through the pipe 10 to the evaporator 1 is regulated by a valve 33 and is at such a rate that a constant average level is maintained in the separator, the level being of course highest at the moment when the valve 21 is about to be opened to withdraw the sodium chlorid, and lowest after the discharge has been effected and the valve is again closed. It must be borne in mind that the level of the liquor in the separator 3 should not at any time be below the outflow opening at 34 through which the desalted liquor passes to the evaporator 4, and the dimension and arrangement of the separator and other parts must be made accordingly.

When an apparatus has been in operation for some time it may be found that some of the tubes in the calandria have become coated with salt which must of course be removed sooner or later. This can be done very easily by simply passing clean water or steam through the tubes but as such cleaning, although not occupying a long time, would nevertheless interrupt the process, we contemplate providing two or even more calandria to each separator with the necessary valves and connections. In the drawing we have lndicated 1n brokenv lines a second calandrla for each separator so that one may be in use while the other is being washed or repaired.

The water of condensation from all the calandria is pumped into a suitable tank 35 through the pipes 36 and is intended to be used to wash out the tubes of the calandria from time to time, the resultant llquor or wash water being then delivered into a tank 37. If not too heavily salted asthe result of one suchwashing, this wash water may be again used to wash out the tubes.

The liquor extracted by the hydro-extractor and which it was stated hereinbefore is pumped back into the first evaporator,

usually contains other salts in solution than sodium nitrate, for example magnesium chlorid and calcium chlorid.

WVhile a small percentage of these will not matter in the least there will arrive a time when by reason of an accumulation of these chlorids (or other impurities) the liquor extracted will be almost saturated therewith and consequently a deposition of such chlorids (or other impurities) with the sodium nitrate would take place in the separator 6.

To avoid this we occasionally pump the extracted liquor into a special tank and after recovering the sodium nitrate in any suitable manner, the water now containing only impurities is thrown away.

From the foregoing description it will be seen that when once the apparatus is in full operation, a constant or continuous separation of sodium chlorid takes place in the evaporator l and simultaneously therewith a constant or continuous crystallization of 7 sodium nitrate in the evaporator 4:.

By working the two evaporators 1 and 4 at different pressures it is possible to evap-v orate the liquor in the latter by the heat contained in the vapors given off in the former so that there is very little waste of heat.

By reason of the fact that in our process both the separation of the sodium chlorid and the recovery of the sodium nitrate are brought about while maintaining the liquor at boiling point (which is of course difi'er-' ent in the two evaporators) we are enabled to recover the sodium nitrate in a few hours from the time of starting as compared with the period of" at least 14: to 16 days which is necessary in the present day processes wherein the crystallization of the sodium nitrate is brought about by allowing the liquor to cool down. This fact alone very considerably reduces the ultimate cost of the nitrate of soda recovered.

By the perfect control that is possible in the regulation of the temperature of the liquor in our apparatus the sodium nitrate can be recovered either practically free from sodium chlorid or together with any desired and definite percentage thereof. This, as already pointed out, is notpossible with the processes heretofore in use on a commercial scale.

\Vhat we claim as our invention and desire to secure by Letters Patent is 1. A two-stage evaporation rocess for recovering sodium nitrate from iquors containing it and sodium chlorid in solution maintaining said liquor at such tempera-- ture transferring the hot supernatant and desalted liquor to another evaporator and therein immediately subjecting it to further evaporation by boiling to thereby bring about the crystallization of the sodium nitrate.

2. A process for recovering sodium nitrate from liquors containing it and sodium chlorid in solution, consisting in insolubilizing the sodium chlorid by concentrating and raising the boiling point of the liquor to the point at which sodium ohloridis in, soluble in the nitrate solution and, after allowing the sodium chlorid to settle, sub jecting the desalted liquor to further evapo-- ration by boiling.

'3. A process for recovering sodium nitrate from liquors containing it and sodium chlorid in solution, consisting in desalting the liquor by concentrating it and raising its boiling point at one pressure to substantially the point at which sodium chlorid is insoluble in the nitrate solution, and thereafter subjecting the desalted liquor to further evaporation by' boiling at a desired less pressure.

4. A process for recovering sodium nitrate from liquors containing it and sodium chlorid in solution, consisting in insolubilizing the sodium chlorid by concentrating and raising the boiling point of the liquor in an evaporator working at one pressure to substantially the point at which sodium chlorid is insolublejn the nitrate solution and .thereafter transferring the desalted liquor to another evaporator working at a desired less pressure and therein subjecting the liquor to further evaporation by boiling.

5. A process for recovering sodium nitrate from liquors containing it and sodium chlorid in solution, consisting in a two-stage evaporation of the liquor, during the first of which the liquor is concentrated in an evaporator working at one pressure and its boiling point raised to the point at which sodium chlorid is insoluble in the nitrate solution, and during the second of which, the desalted liquor, after having been transferred to another evaporator workingat a less pressure, is subjected to further evaporation by boiling, the necessary heat being provided by the hot vapor given off in the first evaporator,

6. A process for recovering sodium nitrate from liquors containing it and sodium trate solution and thereafter subjecting the other evaporator and therein subjecting it desalted liquor to further evaporation by to further evaporation by boiling. boiling. In testimony whereof We have signed our 7. A process for recovering sodium ninames to this specification in the presence 15 5 trate from llquors containmg 1t and sodium of two subscribing Witnesses.

chlorid in solution, consisting in concentrating the liquor in a suitable evaporator, rais- E E 8 ing the boiling point of the liquor to a temperature at which the sodium chlorid -l'Vitnesses: 10 Will be rendered insoluble, transferring the A. EJWILLIAMS,

hot supernatant and desalted liquor to an- M. PoLLoN. 

